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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

COLUMBIA GLACIER
279
The lobate ice margin spreads out in the depressions between a series of rock along the eastern margin and rides up on the sides of the hills. The position of the 1892 advance was clearly marked along this whole margin but the ice did not extend quite up to the windrows of prostrate and inclined trees, though since 1899 it had advanced several hundred feet and overridden the sites of several small ponds.
During the last week in June and the first week in July, 1910, we observed the conditions along this lobate east margin in detail. There had been an advance of two or three hundred feet so that the ice was up to or just beyond the 1892 maximum, having completely covered the barren zone.
Just east of the terminal moraine that fronts the eastern ice cliff, which was tidal ten months before, was an area of low treeless morainic hills made up of glacial deposits, in places resting upon rock ledges. In 1910 the glacier was ploughing forward into this deposit and had developed a series of minor lobes. There was no pronounced terminal moraine at the ice edge but there was a low push moraine, two or three feet high in places. The glacier was evidently crowding the whole deposit forward against the rock slope to the southeast. Locally near the ice edge folds of peaty material had been rolled up, but throughout most of the area, there was no covering of peaty soil upon the glacial till and, therefore, no folding.
The mass of till had been pushed forward against the rock slope to the south and had then bulged upward, damming back the stream into a new lake (Fig. 34), at whose outlet there was a foaming waterfall 10 feet high in June, 1910, where there was none in August, 1909. This lake rose high enough to submerge trees, shrubs, moss, and turf that in the fall of 1909 were growing high above the water. Other evidence of the bulging upward of this till mass was the presence of marine shells and seaweed on top'of a hillock beside the waterfall, 24 feet above high tide. These shells were within the reach of the tide ten months before, for at high tide in 1909 salt water went up the river a quarter mile above the site of the waterfall of June, 1910. It was a narrow estuary at high tide and a narrow stream at low water.
The surface of this whole disturbed area was gashed by gaping cracks, most of which were shallow, due to the compression and upward swelling of the till mass under the thrust from the advancing glacier. A majority of the cracks extended at right angles to the front of the glacier, that is parallel to the direction from which the strain was being
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FIG. 34.   PORTION OF LOBATH EASTHBN MABGOT or COLUMBIA GLA.CDDB, SHOWING NEW LAKH FOBMBD BETWEEN AUGUST,
1909, AND JUNB,  1910.
This is on site of "large glacial river" on Grant and Higgins map (Fig. 31) made in June, 1909. At o was an abandoned river course SA feet above high tide in June, 1910, and marine fossils were found on top of hill. Dotted lines enclose area of cracks near waterfall where advance was pushing up a dam of glacial till in 1910.